The exhaust gas of diesel engines, gasoline engines, and other internal combustion engines includes, for example, carbon monoxide (CO), unburned fuel (HC), nitrogen oxides (NOX), particulate matter (PM), and other constituents. The internal combustion engines are mounted with exhaust purification systems for removing these ingredients.
The NOX which is contained in the exhaust gas is purified by reduction. To purify the NOX which is contained in the exhaust gas, sometimes an NOX selective reduction catalyst which can selectively reduce NOX is arranged in the engine exhaust passage. The NOX selective reduction catalyst can reduce the NOX which is exhausted from the engine body by feeding a reducing agent in a predetermined temperature region.
Japanese Patent Publication (A) No. 2003-529699 discloses a system which includes a first catalyst which is arranged downstream of the engine body and a second catalyst which is arranged downstream of the first catalyst wherein the first catalyst reduces the NOX when in a first temperature range and the second catalyst reduces the NOX when in a second temperature range. This publication discloses introducing a reducing agent to the catalyst where the temperature of exhaust gas is within the temperature range of catalyst activity among the two catalysts arranged in series.